1. Field of the Invention
Embodiments of the present invention are directed to engine exhaust catalysts and more particularly to engine exhaust catalysts containing copper-ceria.
2. Description of the Related Art
Copper or copper oxide on cerium oxide, which are referred to as copper-ceria catalysts have been employed in various applications, such as total oxidation of carbon monoxide and methane, methanol, the water-gas shift reaction, and oxidation of phenol.
In U.S. Pat. No. 3,819,535, an improved catalyst for oxidation of hydrocarbons and carbon monoxide present in engine exhaust gases is disclosed where the catalyst is prepared by depositing copper oxide and ceria on an alumina support that has been stabilized against shrinkage. The improved catalyst according to U.S. Pat. No. 3,819,535 provides high activity for carbon monoxide combustion as well as sufficient activity to oxidize hydrocarbons. Other advantages that are cited in the patent include a low ignition temperature for carbon monoxide and maintaining high activity even after extended exposure to high temperatures.
In U.S. Pat. No. 4,996,180, a catalyst containing an intimate mixture of copper oxide and ceria that is suitable for oxidation or reduction of chemical feedstocks, low temperature water gas shift, methanol synthesis, and controlling engine exhaust emissions is disclosed. A key feature of this catalyst is that the weight of the copper oxide is less than the weight of the ceria. Experiments disclosed in U.S. Pat. No. 4,996,180 show inferior oxidation and reduction performance when the copper content is very high.
U.S. Pat. No. 7,220,692 shows copper-ceria applied to selective catalyst reduction (SCR). This patent teaches the addition of ceria as a stabilizing oxide to zeolite-based materials in SCR applications as a way to combat the loss of catalytic activity of the zeolite-based materials under wet conditions. According to U.S. Pat. No. 7,220,692, the addition of ceria to zeolite-based materials, in particular Cu-ZSM-5, improves hydrothermal stability of Cu-ZSM-5 so that the catalytic activity of Cu-ZSM-5 is sustained even under wet conditions.
Copper-ceria catalysts have also been employed in cigarette filters to catalyze the oxidation of carbon monoxide at low temperatures and reduce the amount of carbon monoxide in cigarette smoke. In U.S. Pat. No. 6,857,431, copper oxide nanoparticles and/or copper nanoparticles are combined with ceria nanoparticles to reduce the amount of carbon monoxide in cigarette smoke. This patent recognizes the application of such catalysts to vehicle exhaust emissions systems of automobiles and diesel engines and cold starting systems of automobile engines.
As has been recognized in the art, copper-ceria catalysts are attractive because they are cost effective relative to catalyst containing precious metals such as platinum (Pt), palladium (Pd), and the like, and are good oxidation catalysts at low temperatures. Despite these advantages, their use in automotive applications has been very limited. An investigation of most commercial emission control systems of today will reveal that copper-ceria is not employed as a catalytically active component.